fix($core): preserve original meta charset and viewport (vuejs#2332)

Author: sakihet

Diff for: packages/@vuepress/core/lib/client/root-mixins/updateMeta.js

@@ -37,8 +37,14 @@ export default {
       const pageMeta = this.$page.frontmatter.meta || []
       // pageMetaTags have higher priority than siteMetaTags
       // description needs special attention as it has too many entries
-      return unionBy([{ name: 'description', content: this.$description }],
-        pageMeta, this.siteMeta, metaIdentifier)
+      const ary = [{ name: 'description', content: this.$description }]
+      if (document) {
+        ary.push(
+          { charset: document.querySelector('meta[charset]').getAttribute('charset') },
+          { name: 'viewport', content: document.querySelector('meta[name=viewport]').content }
+        )
+      }
+      return unionBy(ary, pageMeta, this.siteMeta, metaIdentifier)
     }
   },